Thanks to the Argo ocean observation system, a network of autonomous floats that was deployed in the early 2000s, we now have much greater capacity for monitoring ocean temperatures. At the same time, researchers have been steadily improving our understanding of long-term ocean heat content trends by correcting for biases and errors in previous analyses of the existing pre-Argo data.

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The result is that it is now possible to reconstruct the ocean temperature record all the way back to 1960 with a high degree of accuracy, according to Dr. Lijing Cheng of the Institute of Atmospheric Physics at the Chinese Academy of Sciences. Cheng led a research team that combined new observations enabled by the Argo network with the adjusted data from the past to show that ocean warming is accelerating at a rapid pace and will continue to do so unless we draw down greenhouse gas emissions. The team's findings were detailed in a paper published in Science today.

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The mean speed of ocean warming over the past 60 years, from roughly 1958 to 2017, was 5.46 zettajoules per year, Cheng says. That's more ocean warming than was reported in the Intergovernmental Panel on Climate Change's Fifth Assessment Report, published in 2013. The oceans will warm at an even more rapid pace over the next 60 years, with the mean speed of ocean warming projected to be 23.78 zettajoules per year. "So the speed of ocean warming from 2020 to 2081 is roughly 4.4 times higher than that of the past 60 years," Cheng says.

The seas absorb some 93 percent of the energy imbalance created by the increasing concentrations of greenhouse gases in Earth's atmosphere. While climate models project continuous warming of the oceans throughout the 21st century, how much ocean waters actually heat up depends on the actions taken by humans to rein in the greenhouse gas emissions causing global climate change.

If no actions are taken and we proceed with "business as usual," the upper ocean (above a depth of 2,000 meters) will warm by 2,020 zettajoules by 2081–2100, six times more than the total ocean warming recorded over the past 60 years, according to the study. If we were to meet the emissions reductions targets that countries committed to under the Paris Climate Agreement, however, we could cut total ocean warming within that timeframe nearly in half to about 1,037 zettajoules.

As Cheng points out, ocean warming has broad and serious impacts on ocean ecosystems and human society alike. For instance, increased temperatures cause thermal expansion of water and thus contribute to sea level rise—about one-third of the total sea level rise in the 20th century was due to ocean warming, Cheng says. "The resulting sea level rise exposes coastal fresh-water supplies to salt-water intrusion, makes communities more susceptible to storm surge, and threatens coastal infrastructure."

Warming also leads to declines in the amount of oxygen that can dissolve in seawater and alters patterns of global ocean circulation, which affects the mixing of oxygen-rich surface waters with the more oxygen-poor waters at deeper depths. This has severe implications for marine wildlife that are continuously consuming oxygen "essentially everywhere in the ocean," Cheng says. Ocean warming contributes to the bleaching and death of corals, which take up about 0.1 percent of Earth's ocean area but are home to a quarter of all marine life, as well, because "they simply can't tolerate high temperatures and ocean acidification."

Cheng says that he and his colleagues plan to continue monitoring ocean warming and will release ocean heat content data later this month showing that 2018 was the warmest year on record for the global ocean. (Cheng was also the lead author of a 2018 study that determined 2017 was the hottest year for oceans yet, at least up to that point.) But he argues that, in order to get the complete picture of how global warming is affecting our planet, we need more ocean monitoring systems in coastal waters, marginal seas, polar regions, and in the ocean at depths below 2,000 meters.

"The current observing capability in those regions are still limited. But these regions directly connect [with] human communities and are more vulnerable to climate change," Cheng says. "Global warming is actually ocean warming, because ocean stores more than 90 percent of the heat trapped by greenhouse gasses. Therefore, monitoring ocean warming is to know how much our Earth is warming. If not going below 2,000 meters, we will never know how much the ocean, as a whole, is warming."

Cheng and his co-authors note in the Science article that there is also a need to continue refining our methods for observing and analyzing ocean heat content in order to make regionally specific projections of future warming. "In addition, the need to slow or stop the rates of climate change and prepare for the expected impacts is increasingly evident," the researchers write.

This story originally appeared at the website of global conservation news service Mongabay.com. Get updates on their stories delivered to your inbox, or follow @Mongabay on Facebook, Instagram, or Twitter.

Reports show that the ocean's unique ecosystems are adapting to fluctuations in water temperatures likely caused by global warming, but increasing acidic levels may prove fatal for the world's coral reefs.